Free-space Communication Systems Research Articles

Identification of independent modes in two inputs free space communications system

The study proves the concept of using a system with two inputs laser-beam, modulated independently by helical and conical phase distributions followed by their superposition in the same physical path for free space optical communications. Both phase distributions generate annular intensities with specific geometry according to the modulating parameters. The modes are multiplexed in an emission unit and propagate via laboratory free space as rings with optimized radii, without interfering, to the reading phase mask of the receiving unit. The reading mask acts as matching filter which embeds planar phase information such that to distribute the helical and conical components on mutual perpendicular directions. Identification occurs when the parameters of the reading mask match those of the input modes as predicted by a 2-dim inner product model developed here and confirmed experimentally by the occurrence of maximum intensities in the specific centers. Splitting the matrix-like diffracted beam in enabler and processing branches allows to double check and to identify the matching cases by the properties of the associated histogram without affecting subsequent signal retrieval. Crosstalk levels are examined.

Phase compensation for free-space continuous-variable quantum key distribution.

Large-scale and flexible deployment of quantum networks is possible with reliable free-space quantum key distribution. However, signal fading occurs in free-space channels and causes various adverse effects. Under this circumstance, phase compensation becomes a challenging task for quantum key distribution using continuous variables. Here we investigate the feasibility of implementing phase compensation via simply computing the correlation between transmitted and received data. Demonstration and performance analysis are conducted with real transmittance of a 150-m free-space fading channel; results indicate the applicability of this compensation scheme to free-space quantum communication systems.

Inverse-designed non-reciprocal pulse router for chip-based LiDAR

Non-reciprocal devices such as isolators and circulators are key enabling technologies for communication systems, both at microwave and optical frequencies. Although non-reciprocal devices based on magnetic effects are available for free-space and fibre-optic communication systems, their on-chip integration has been challenging, primarily due to the concomitant high insertion loss, weak magneto-optical effects and material incompatibility. We show that χ(3) nonlinear resonators can be used to achieve all-passive, low-loss, bias-free non-reciprocal transmission for applications in photonic systems such as chip-scale LiDAR. A multi-port nonlinear Fano resonator is used as an on-chip, non-reciprocal pulse router for frequency comb-based optical ranging. Because time-reversal symmetry imposes stringent limitations on the operating power range and transmission of a single nonlinear resonator, we implement a cascaded Fano–Lorentzian resonator system that overcomes these limitations and substantially improves the insertion loss and operating power range of current state-of-the-art devices. This work provides a platform-independent design for non-reciprocal transmission and routing that is ideally suited for photonic integration.

Large-scale optical phased array using a low-power multi-pass silicon photonic platform

Optical phased arrays are a promising beam-steering technology for ultra-small solid-state lidar and free-space communication systems. Long-range, high-performance arrays require a large beam emission area densely packed with thousands of actively phase-controlled, power-hungry light emitting elements. To date, such large-scale phased arrays have been impossible to realize since current demonstrated technologies would operate at untenable electrical power levels. Here we show a multi-pass photonic platform integrated into a large-scale phased array that lowers phase shifter power consumption by nearly 9 times. The multi-pass structure decreases the power consumption of a thermo-optic phase shifter to aPπof1.7mW/πwithout sacrificing speed or optical bandwidth. Using this platform, we demonstrate a silicon photonic phased array containing 512 actively controlled elements, consuming only 1.9 W of power while performing 2D beam steering over a70∘×6∘field of view. Our results demonstrate a path forward to building scalable phased arrays containing thousands of active elements.

不同天气条件对自由空间量子通信系统性能的影响

不同天气条件对自由空间量子通信系统性能的影响

Optical carrier suppression with modified duo binary return to zero and polarization shift keying modulation schemes over free space communication system

Free space optical (FSO) communication has been a subject of interest for researcher and the corporate world since a few decades ago. This has been motivated by the huge benefits associated with this technology. Meanwhile, the generation and all optical up conversion of millimeter wave (mmw) for propagation in a FSO channel has been an important research direction. In this article, a single side band tone mmw generated by the optical carrier suppression (OCS) method has been modulated by modified duo-binary return to zero (MDRZ) and polarization shift keying (PolSK) modulated baseband data. The resulting optical signal was propagated in a FSO cannel subjected to combined weather and atmospheric turbulence induced fading. Later on, a comparative analysis was carried out between hybrid OCS/MDRZ and OCS/PolSK mmw signals propagated over free space. Using the performance metrics such as the bit error rate, quality (Q) factor and the received power among others, the simulation results indicated that hybrid OCS/MDRZ performs better than hybrid OCS/PolSK.

Performance Improvement of FSO Communication System using MIMO Technique

In proposed simulation model, the effect of different MIMO system has been analyzed on performance of free space optical (FSO) system concealed by various climatic conditions like clear, haze and fog. FSO is a free space communication system where space acts as medium between transceivers which provides a strong and efficient method for transmission and reception of information. The effect of different weather conditions has been investigated for range of 1 and 2 km at frequency 193.1 GHz. The proposed MIMO-FSO simulation system has been analyzed for different parameters like Q factor, height of eye, power received and bit error rate. The designed 4X4 MIMO-FSO system provides better results as compared to the 2X2MIMO and SISO-FSO system.

A novel free space communication system using nonlinear InGaAsP microsystem resonators for enabling power-control toward smart cities

Nowadays, the smart grid has demonstrated a great ability to make life easier and more comfortable given recent advances. This paper studies the above issue from the perspective of two important and very useful smart grid applications, i.e., the advanced metering infrastructure and demand response using the instrumentality of a set of well-known scheduling algorithms, e.g., best-channel quality indicator, log rule, round robin, and exponential–proportional fairness to validate the performance. To increase the data transmission bandwidth, a new concept of optical wireless communication known as free-space optical communication (FSO) system based on microring resonator (MRR) with the ability to deliver up to gigabit (line of sight) transmission per second is proposed for the two studied smart grid applications. The range between 374.7 and 374.79 THz frequency band was chosen for the generation of 10 successive-carriers with a free spectral range of 8.87 GHz. The ten multi-carriers were produced through drop port of the MRR. The results show up to 10 times bandwidth improvement over the radius as large as 600 m and maintain receive power higher than the minimum threshold (− 20 dBm) at the controller/users, so the overall system is still able to detect the FSO signal and extract the original data without detection.

Orbital angular momentum demultiplexing with synthetic partial aperture receivers

Long-distance transmitted orbital angular momentum (OAM) cannot be received completely by a single aperture receiver due to the divergence of OAM beams in free space, which results in a challenge for measuring and demultiplexing the multi-OAM modes. We propose a novel approach in measuring multi-OAM modes by using synthetic partial aperture receivers, which could significantly reduce the intensity crosstalk caused by a power leakage. The available OAM modes in a long-distance free-space communication system are greatly increased, and all of the OAM modes can be used for multiplexing or encoding. Furthermore, an OAM demultiplexing array is employed to convert the incident OAM modes into Gaussian-like beams that can be detected by commercial photodetectors with high speed. The proposed approach has great potential applications in long-distance free-space OAM detection and communication.

Simulation and Experimental Verification for a 52 GHz Wideband Trapped Surface Wave Propagation System

Trapped surface wave (TSW) provides a flexible 2-D wireless solution when compared to existing wired and free space communications systems. This work aims to provide the theoretical guidance, supported by simulation techniques and experimental verifications, for 1) designing the wideband and highly efficient rectangular aperture TSW transducers and 2) selecting the best reactive surface impedance for high efficiency. First, a method for computing the TSW excitation efficiency is proposed for the first time and it can be applied to different kinds of reactive surface geometries and transducer apertures. Then, the relation between the TSW excitation efficiency, surface reactance, and aperture height is derived. Then, the maximum excitation efficiency, the corresponding optimal surface reactance, and aperture height are presented. Furthermore, the relation between the TSW angular coverage and aperture width is determined. These studies show that the aperture height mainly determines the TSW excitation efficiencies, while the aperture width controls the TSW angular coverage; hence, the two aperture parameters of a transducer can be independently determined. Finally, an experiment setup, based on the provided guidelines, has been built to demonstrate a 52 GHz wide 3 dB-transmission-bandwidth for high-performance communications systems.

Transmission Characteristics of Adaptive Compensation for Joint Atmospheric Turbulence Effects on the OAM-Based Wireless Communication System

Atmospheric turbulence has an impact on the transmission of electromagnetic vortex waves with an orbital angular momentum (OAM) mode. In this paper, based on the joint atmospheric turbulence model, we examine the influence of atmospheric turbulence on the transmission of electromagnetic vortex waves. First, a mathematical model is established to formulate the transmission characteristics of electromagnetic vortex waves under joint atmospheric turbulence. Subsequently, in order to mitigate the influence of the atmospheric turbulence on the electromagnetic vortex waves, an adaptive compensation on phase is proposed. Finally, we analyzed the transmission performance of an OAM-mode multiplexing system using an adaptive compensation method through the wireless communication channel. By means of numerical simulation, the effect of atmospheric turbulence on the transmission characteristics and mode crosstalk of OAM is analyzed. The simulation results show that electromagnetic vortex waves could perform well on the wireless communication system with a low mode crosstalk, which provides the theoretical support to optimizing the mode division multiplexing technology in a free space communication system.

Zero-error attacks on a quantum key distribution FSO system

Quantum key distribution (QKD) is a method for exchanging keys between two parties (Alice and Bob) with an absolute level of security in the ideal scenario. QKD systems are vulnerable to hacking through secondary emissions of photons by SPAD detectors, known as backflash. In this work, we derive an information leakage probability model for the scenario of QKD in a free space communication (FSO) system, and suggest a method for its minimization.

Experimental investigation of wind and temperature induced scintillation effect on optical wireless communication link

Atmospheric disturbances due to randomly distributed wind flow and temperature results in fluctuation of intensity as well as in the phase of the received light signal, impairing an optical wireless link performance. In this work, turbulence-induced scintillation on Optical Wireless Communication (OWC) is investigated experimentally on the basis of optimized signal wavelength, link length and launched power level. To mimic the outdoor environmental conditions, an indoor chamber equipped with fans and heating coil is made up, which gives manual control of the temperature and wind velocity within the chamber. The received signal bearing data rate of 100 Mbps is measured after traversing through the designed chamber and used to characterize the turbulence strength in terms of scintillation index (S.I.). To establish a long-range OWC link, the atmospheric channel length is increased using round-trip reflection by placing plane front surface mirrors at both ends of the chamber, also front concave mirror is used to collect distributed irradiance onto aperture area of the detector at the receiver. Experimentally, we have tried to characterize the combined effect of the wind and the temperature on the free space communication system. Various communication metrics like bit error rate (BER), signal to noise ratio (SNR) and Q factor have been evaluated and it is reported that the BER performance of an OWC link having wind flow of 15 km/h gets degraded by 109 as compared to still air.

Wideband Ge-Rich SiGe Polarization-Insensitive Waveguides for Mid-Infrared Free-Space Communications

The recent development of quantum cascade lasers, with room-temperature emission in the mid-infrared range, opened new opportunities for the implementation of ultra-wideband communication systems. Specifically, the mid-infrared atmospheric transparency windows, comprising wavelengths between 3–5 µm and 8–14 µm, have great potential for free-space communications, as they provide a wide unregulated spectrum with low Mie and Rayleigh scattering and reduced background noise. Despite the great efforts devoted to the development of mid-infrared sources and detectors, little attention is dedicated to the management of polarization for signal processing. In this work, we used Ge-rich SiGe alloys to build a wideband and polarization-insensitive mid-infrared photonic platform. We showed that the gradual index change in the SiGe alloys enabled the design of waveguides with remarkably low birefringence, below 2 × 10−4, over ultra-wide wavelength ranges within both atmospheric transparency windows, near wavelengths of 3.5 µm and 9 µm. We also report on the design of a polarization-independent multimode interference device achieving efficient power splitting in an unprecedented 4.5-µm bandwidth at around 10-µm wavelength. The ultra-wideband polarization-insensitive building blocks presented here pave the way for the development of high-performance on-chip photonic circuits for next-generation mid-infrared free-space communication systems.

Experimental demonstration of 4-PAM for high-speed indoor free-space OW communication based on cascade FIR-LMS adaptive equalizer

Inter-symbol interference (ISI) is one of the key problems that seriously limit the transmission data rate in optical communication system. To eliminate ISI and further improve the system performance, we design a specific cascade finite impulse response-least mean square (FIR-LMS) equalizer. To the best of our knowledge, it is the first time this kind of equalizer is employed for high-speed indoor free-space optical wireless (OW) communication system. After LD driver printed circuit board (PCB) validated the design, a special miniaturized and inexpensive LD, and a spatial photodetector are employed at the transmitter and receiver, respectively. Based on these, we experimentally investigate the scheme of 4-pulse amplitude modulation (4-PAM) optical signal combined with cascade FIR-LMS adaptive equalizer in an indoor free-space OW communication system. The convergence performance between LMS and FIR-LMS is compared, the bit error rate (BER) performances of FIR, LMS and FIR-LMS equalizer at different propagation distances are also demonstrated. With the utility of the proposed equalizer, the OW communication system can successfully achieve the bit rate 10 Gbit/s transmission over 500 cm indoor free-space link distance. The BER significantly below the 7% forward error correction (FEC) limit of 3.8×10−3, which is lower than LMS by more than an order of magnitude.

Demonstration of analyzers for multimode photonic time-bin qubits

We demonstrate two approaches for unbalanced interferometers as time-bin qubit analyzers for quantum communication, robust against mode distortions and polarization effects as expected from free-space quantum communication systems including wavefront deformations, path fluctuations, pointing errors, and optical elements. Despite strong spatial and temporal distortions of the optical mode of a time-bin qubit, entangled with a separate polarization qubit, we verify entanglement using the Negative Partial Transpose, with the measured visibility of up to 0.85$\pm$0.01. The robustness of the analyzers is further demonstrated for various angles of incidence up to 0.2$^{\circ}$. The output of the interferometers is coupled into multimode fiber yielding a high system throughput of 0.74. Therefore, these analyzers are suitable and efficient for quantum communication over multimode optical channels.

Correction of Wavefront Tilt Caused by Atmospheric Turbulence Using Quadrant Detectors for Enabling Fast Free-Space Quantum Communications in Daylight

Low photon-level signals used in most free-space quantum communication systems require a narrow field of view in the receiver to minimize the amount of background noise coupled into the single photon detectors. This can be achieved through beam tracking techniques, which compensate atmospheric effects, such as beam wander, in the receiver, reducing the long-term beam area. However, reducing the diameter of this area below a few microns, typically necessary to achieve a low level of solar background noise and successful daylight quantum transmission, require fine tracking precision and diffraction-limited optics. We demonstrate that this can be done with standard voice-coil fast steering mirrors and cheap commercially-available quadrant detectors. Two correcting strategies (open and closed loop) are experimentally tested and analyzed for their applicability in metropolitan (~km range) free-space quantum communications. The area containing the random fluctuations of the beam centroid caused by atmospheric turbulence at the focal plane of the receiver was reduced by a factor of 4 with an open-loop configuration, and up to a factor of nine with a closed loop configuration. This is equivalent to a reduction in the quantum bit error rate caused by background solar noise of up to one order of magnitude, which, combined with spectral filtering techniques, enable the possibility of fast daylight quantum key distribution.

Unidirectional waveguide grating antennas with uniform emission for optical phased arrays.

We demonstrate millimeter-scale optical waveguide grating antennas with unidirectional emission for integrated optical phased arrays. Unidirectional emission eliminates the fundamental problem of blind spots in the element factor of a phased array caused by reflections of antenna radiation within the substrate. Over 90% directionality is demonstrated using a design consisting of two silicon nitride layers. Furthermore, the perturbation strength along the antenna is apodized to achieve uniform emission for the first time, to the best of our knowledge, on a millimeter scale. This allows for a high effective aperture and receiving efficiency. The emission profile of the measured 3mm long antenna has a standard deviation of 8.65% of the mean. These antennas are state of the art and will allow for integrated optical phased arrays with blind-spot-free high transmission output power and high receiving efficiency for LIDAR and free-space communication systems.

Instrumentation limitation on a polarization-based entangled photon source

Free-space optical communication is hindered by turbulence resulting in spatial modal dispersion of the optical beam. Here we mimic in the laboratory the far-field turbulence effects on entangled photons in the polarization basis. We make use of a diffractive optical element to simulate turbulence distortions and measure the entanglement as a function of the turbulence strength. We show that the standard method of coincidence detection using single-mode-fiber coupling to single-photon counters results in spatial mode dependence of entanglement even though it is not measured with spatial modes. We find that the overall coupling inefficiency of the detected coincidence caused by spatial mode dispersion in free space can be corrected by the use of a multimode fiber. Our results suggest that care is required in the choice of a detection system for free-space quantum communication systems.

Multipath Effects in Millimetre-Wave Wireless Communication using Orbital Angular Momentum Multiplexing.

Electromagnetic waves carrying orbital angular momentum (OAM) have been used for mode division multiplexing in free-space communication systems to increase both the capacity and the spectral efficiency. In the case of conventional wireless communication links using non-OAM beams, multipath effects caused by beam spreading and reflection from the surrounding objects affect the system performance. This paper presents the results of analysis, simulations, and measurements of multipath effects in a millimetre-wave communication link using OAM multiplexing at 28 GHz. Multipath-induced intra- and inter-channel crosstalk, which are caused by specular reflection from a plane parallel to the propagation path, are analysed and measured. Both the simulation and the experimental results show that an OAM channel with a high OAM number ℓ tends to suffer from both strong intra-channel crosstalk and strong inter-channel crosstalk with other OAM channels. Results of the analysis show that this observation can be explained on the basis of both the properties of OAM beam divergence and the filtering effect at the receiver, which is associated with the spiral wavefront of OAM beams.